This invention relates to control circuits and more particularly, to circuits for controlling light emitting diodes and the like.
One of the many uses for light emitting diodes (LEDs) is to provide a source of light to trigger a photosensitive semiconductor device into conduction. An example of such a use is described in copending U.S. patent application Ser. No. 164,224, filed June 30, 1980, entitled "IMPROVED ZERO VOLTAGE SWITCHING AC RELAY CIRCUIT," and assigned to the assignee of the present invention.
The above referenced application discloses a circuit including two series connected LEDs which are optically coupled, respectively, to two photosensitive silicon controlled rectifiers (SCRs). When the LEDs are energized by the application of an input signal, they generate light which causes the SCRs to conduct. The SCRs, in turn, control the application of power to a load.
For proper operation, this type of circuit and others like it usually require that the LEDs be operated at a current level ranging between specified levels of minimum and maximum current. Operation of the LEDs outside of the specified range may result in improper circuit operation or component failure. Generally, the level of current supplied to the LEDs is determined by the value of a fixed resistor connected in series between the LEDs and the input signal source.
Circuits of the type described above, usually designated as solid state relay circuits, find widespread use as interfaces between low level logic circuits and high power circuits in which the output signal from the logic circuit is used to energize the LEDs. However, many solid state relay circuits require that the LEDs be energized at a current level which exceeds the output current capability of some types of logic circuits. For example, complementary metal oxide semiconductor (CMOS) logic circuits are generally not capable of supplying more than one or two milliamperes of output current while many relay circuits require a minimum LED current of five milliamperes or more for proper operation. Accordingly, it is necessary to provide a separate buffer circuit between a CMOS logic circuit and the LEDs to provide the necessary drive current.
In some applications of solid state relay circuits it is desirable to energize the LEDs with an input signal which varies over a wide range of voltages. For example, it may be desirable to energize the LEDs from a voltage source ranging from as little as five to as much as thirty volts. However, many prior art circuits cannot be so operated because they employ a fixed resistor to establish the proper level of LED operating current for a predetermined input voltage; and, if operated with a different input voltage require a different resistor to establish the proper level of operating current.
Many prior art circuits lack the capability to ensure that the LED current is above a specified minimum level. This is so because the level of LED current in the prior art circuits is a function of both the value of the fixed resistor and the magnitude of the input voltage. Thus, for example, if the resistor is chosen to establish the minimum required LED current at an input voltage of five volts, the application of a lower input voltage will result in insufficient current flow to the LED.
Accordingly, it is an object of the present invention to provide a new and improved circuit for controlling a light emitting diode.
It is another object of the invention to control a light emitting diode over a wide range of input voltage.
It is still another object of the invention to control a light emitting diode in direct response to the output signal from a low power logic circuit such as a CMOS logic circuit.
It is yet another object of the invention to control a light emitting diode in a manner which ensures that the diode is operated at a predetermined current level.